The National Imagery and Mapping Agency (NIMA) maintains a worldwide GPS Monitor Station Network (MSN). We are in the process of studying carrier-phase frequency transfer as one method for understanding the overall network performance, and for potential use in testing future GPS receivers. We have developed processing software that uses 30-second dual frequency data in a continuous sequential estimator, which allows unlimited forward processing as well as optional backwards (fixed-interval) smoothing. States in the estimation include tropospheric and ionospheric delays, corrections (assumed small) to the station positions, and double-differenced carrier-phase biases. Satellite positions are taken from either precise or broadcast ephemerides. We have tested our process using both zero-baseline datasets and benchtop tests. Our initial results indicate that instabilities seen in the estimated time offsets are dominated by temperature variations coupling to the GPS receiver used.